The present technology relates to adjustable lenses used in a lens system, and more specifically, to systems and methods for reducing temperature induced drift effects on a micro-fluidic or liquid lens used in a vision system.
Vision systems have been developed for many different applications. For example, machine vision systems have been developed for reading bar codes and other types of symbols placed on packages or products to obtain information there from. Other machine vision systems have been developed for inspecting manufactured parts for features/characteristics.
Many vision systems include a camera for obtaining images of symbols or items to be imaged. A processor receives the images and extracts information that can then be used to perform one or more vision processes. In many applications, the distance between a camera sensor and a symbol or item to be imaged can vary between uses. In these cases, in order to obtain useful images, i.e., images from which data required to complete machine vision processes can be extracted, an adjustable lens and/or auto-focus system is often provided. In these cases, when the system is activated to perform a vision process, the lens and auto-focus system automatically focus the lens so that a clear image of the symbol or item to be imaged is generated on the camera sensor. After the focusing process is completed, a clear image of the symbol or item to be imaged is obtained and is processed to complete the vision process.
One type of adjustable lens that can be used in a machine vision system is a liquid lens. Liquid lenses are constructed of one or more fluids of different refractive indexes, and can be varied by controlling the meniscus, or surface of the liquid. In one type of liquid lens, for example, two fluids are contained in a tube with transparent end caps. The first is an electrically conducting aqueous solution, and the second is a non-conducting oil. The interior of the tube is coated with a hydrophobic material, which causes the aqueous solution to form a hemispherical lens which can be adjusted by applying a DC voltage across the coating to decrease its water repellency in a process called electrowetting. Electrowetting adjusts the surface tension of the liquid, which changes the radius of curvature and adjusts the focal length of the lens. Several liquid lens configurations utilizing an electrowetting process are known.
Another type of adjustable liquid lens utilizes an electrical/mechanical actuator system to induce movement to adjust the focus of the lens. For example, a voice coil type adjustable lens has a ring shaped voice coil actuator that presses onto a transparent membrane serving as a transparent sidewall of a container. The container is filled with a transparent liquid. A current applied through the actuator induces the actuator to apply a force to deform the membrane into a convex shape. The convex shape acts as the lens, and can be adjusted by adjusting the current.
Liquid lenses are extremely versatile, providing a highly variable focal length, and some without the need for moving parts. Liquid lenses, however, are inherently subject to undesirable changes in focal length (referred to herein as drift) due to temperature changes and aging of the liquids in the lens. Temperature and aging can, for example, alter the refractive index of the liquids, or the dielectric constant, thereby changing the focal length. For example, when small symbols are imaged at a fixed large distance, a temperature drift of the lens will cause blur in the image and decrease reading performance. This undesirable drift causes the liquid lens at a first temperature to have a first focal length, and the same liquid lens at a second temperature would have a second focal length different from the first focal length.
For adjustable lenses that use a current applied through the actuator to adjust the focus of the lens, the current applied through the actuator not only heats the actuator, but the lens heats up as well. Undesirably, this causes the temperature of the lens to vary with the applied control current. At large optical power (close object distances) the lens will heat up more than when used at small optical power (large object distance) due to the higher current need for the larger optical power.
Attempts have been made to compensate for liquid lens drift. These attempts measure the thermal behavior of the liquid lens during a calibration process, and then compensate the lens at normal operation based on the measured thermal behavior by adjusting the liquid lens driver voltage or current. This not only requires a time consuming calibration process for each lens, but the measured thermal behavior is made based on a typical drift behavior during calibration, which has limited accuracy.
Therefore, when using a variable lens in applications that induce changes in the temperature of the lens, the focusing of the variable lens will produce different results at different temperatures. For these applications, other systems and methods must be used in an attempt to maintain a more consistent focal length and a sharper resulting image. The present technology addresses solutions to these issues.